In the car industry, in particular, it is conventional for manufacturers to require electronic systems fitted with microprocessors to consume as little electricity as possible. That is why such a microprocessor is typically left in a dormant condition so long as it is not required to perform any operation. Its electricity consumption under such conditions is extremely low (about 10 microamps).
In addition, it is now common practice to use such a microprocessor to perform a plurality of functions by calling different specific subprograms contained in its non-volatile memory (ROM).
In order to "wake up" the microprocessor when a given subprogram is to be run, a pulse is applied either to the microprocessor's reset input terminal RESET, or to its interrupt input terminal INT. In most cases, these are the only two terminals accessible from the outside to act on the operation of the microprocessor.
In entirely conventional manner, a reset pulse is applied to the microprocessor whenever the system is switched on.
Thus, in theory, only the terminal INT is accessible for acting on the operation of the microprocessor from the outside.
When a plurality of units in the system are associated with the microprocessor and need to be able to "wake it up", they must therefore share access to the terminals RESET and INT. Unfortunately, the microprocessor cannot then distinguish immediately on being "woken up" between the various possible origins of this particular "awakening", even though it would be desirable to branch without loss of time to the appropriate subprogram depending on which unit has "woken it up".
The present invention seeks to mitigate these drawbacks of the prior art and to provide a method enabling a microprocessor to distinguish between a reset pulse associated with the system being switched on and a reset pulse having a different origin and to do this quite simply and cheaply.